In a chromatograph mass spectrometer in which a liquid chromatograph or gas chromatograph is combined with a mass spectrometer, i.e. in LC/MS or GC/MS, mass spectra for various components eluted from an LC or GC column with the passage of time can be acquired by repeating a mass spectrometric analysis (MS analysis) over a previously set m/z range.
In the case of a tandem mass spectrometer or an IT-TOF mass spectrometer having an ion trap and time-of-flight mass spectrometer unit, an MS/MS analysis can also be performed in addition to the MS analysis. In these types of mass spectrometers, a technique called the “DDA (data dependent analysis)” or “automatic MS/MS (auto-MS/MS)” is commonly used, in which a precursor ion to be used in the MS/MS analysis is automatically extracted from an MS spectrum obtained by the MS analysis (see Patent Literature 1 or 2).
The DDA is often used to identify unknown compounds in a sample. In this method, a plurality of compounds which may possibly be contained as the unknown compounds are selected as the target compounds with reference to a database which holds the retention time, MS spectrum data and MS/MS spectrum data for a large number of compounds. After that, an analysis schedule is determined as follows:
As one example, the following description assumes that three compounds have been selected as the target compounds: compound A (retention time, 2.5 min), compound B (retention time, 3.5 min) and compound C (retention time, 5.5 min). Initially, for each compound, a mass spectrometric analysis for detecting ions produced from the compound concerned is scheduled for an analysis period having a predetermined width centering on the retention time of the compound (this analysis is hereinafter called the “parent event”). For example, for compound A, an MS analysis (scan analysis) for scanning a mass-to-charge ratio range from m/z=10 to 1000 is scheduled for an analysis period of 0.0-5.0 min. For compound B, an MS analysis for scanning a mass-to-charge ratio range from m/z=1000 to 5000 is scheduled for an analysis period of 1.0-6.0 min. For compound C, an MS analysis for scanning a mass-to-charge ratio range from m/z=5000 to 10000 is scheduled for an analysis period of 3.0-8.0 min. FIG. 1 shows an analysis schedule in which those conditions are set.
Subsequently, the sample is introduced into the chromatograph and the analysis is initiated. After an MS spectrum is obtained by the first measurement performed under the mass spectrometric conditions for compound A, each mass peak which matches the previously set conditions (such a peak is hereinafter described as “satisfying the event conditions”) is extracted from one or more mass peaks which have appeared on the MS spectrum. For example, the “previously set conditions” may require that the peak should have an intensity equal to or greater than a previously set threshold and/or its location should correspond to the mass-to-charge ratio of an ion characteristic of compound A. After that, an MS/MS analysis in which an ion corresponding to the extracted mass peak is designated as the precursor ion is performed (this analysis is hereinafter called the “child event”) to obtain a mass spectrum of the productions.
An analysis which is performed in the case where three, five and two mass peaks satisfying the event conditions have been respectively found in the parent events related to compounds A, B and C is hereinafter described with reference to FIGS. 2A-2C.
Within the analysis period of 0.0-1.0 min, only the parent event for compound A (this event is labelled as “Pa.A” in the figures) is previously scheduled; three child events 1-3 (which are labelled as “C.A1”, etc., in the figures) are added based on the mass spectrum obtained by performing the “Parent A” event. During this analysis period, the “Parent A” event and “Child A1-A3” events are repeated in the mentioned order (FIG. 2A). A measurement in which each of this series of events is executed one time is called the “single measurement”, and the period of time required for the single measurement is called the “loop time” (for example, see Patent Literature 3). The measurement data of each event are obtained at intervals of time equal to the loop time.
Within the analysis period of 1.0-3.0 min, the parent events for compounds A and B are previously scheduled, and “Child A1-A3” events and “Child B1-B5” events are added based on the mass spectra obtained by performing those parent events. During this period, the “Parent A” event, “Child A1-A3” events, “Parent B” event, and “Child B1-B5” events are repeated (FIG. 2B). Similarly, during the analysis period of 3.0-5.0 min, the “Parent A” event, “Child A1-A3” events, “Parent B” event, Child B1-B5” events, “Parent C” event, and “Child C1-C2” events are repeated (FIG. 2C).
After the series of measurements has been completed, the mass spectrum data of the product ions obtained in each child event are compared with the MS/MS spectrum data of each target compound registered in the database, and an unknown compound contained in the sample is identified based on the degree of matching of those data. Additionally, a chromatogram is created from the data obtained from each parent event, and the quantity of the identified unknown compound is determined from the chromatogram.